Such a control circuit is used in a wide variety of electrical applications. For example, such a control circuit is typically used with one or more lamps to control the illumination of the lamp or lamps from a switch having an "ON" and an "OFF" position. In such an application the control circuit senses when the switch is in the "ON" position and in response thereto produces a signal to enable the lamp or lamps to produce illumination, and the control circuit senses when the switch is in the "OFF" position and in response thereto produces a signal to disable the lamp or lamps from illuminating.
Thus, such a control circuit provides control of the lamps in one of two conditions: full illumination or no illumination, depending on whether the switch is instantaneously in the "ON" position or "OFF" position respectively.
In some applications it is desired to control the lamps or lamps in a third condition: for example, at a level of illumination intermediate between full illumination and no illumination. In such an application it has heretofore been necessary to use either an alternative switch having three positions (for example, an "OFF" position, an "intermediate-ON" position and a "full-ON" position) instead of the two-position switch described above, or to use a further two-position switch in addition to the two-position switch described above in order to provide an additional switch position for the intermediate illumination condition of the lamp or lamps. In either case, whether a substitute switch or an additional switch is used to provide the additional switch position for intermediate illumination, additional wiring is typically required to connect the additional switch position to the control circuit.
If it is desired to "retro-fit" lighting having abilities of both "full-ON" and "intermediate-ON" (often referred to as "dimmable" lighting) as well as "OFF" into an existing installation having just "ON" and "OFF" abilities as described above, this will thus require the installation of a different or an additional switch and also require the installation of additional wiring, which will significantly increase the cost of the retro-fitting exercise if the switch or switches are wall-mounted and the additional wiring needs to be hidden within the wall and/or ceiling, as is typically the case.